In a refrigeration cycle and the Rankine cycle, lubrication in an atmosphere of a fluorine compound-based refrigerant (lubrication with a liquid refrigerant) is generally performed. As a lubricating oil (base oil) suitable for lubrication with a liquid refrigerant, a base oil such as polyoxyalkylene glycol, ester, polyvinyl ether and alkyl benzene, which is compatible with a refrigerant, is used (see, for instance, Patent Literatures 1 and 2).
Since a turbo machine is operated at a high speed, lubrication of a bearing therein generally requires use of a lubricating oil in order to maintain cooling performance and lubricity of the bearing. When a high-speed bearing is used in an atmosphere of a refrigerant in the same manner as in a refrigeration cycle and the Rankine cycle, an appropriate lubricating oil designed with allowance for viscosity reduction by atmospheric gas is generally applied.
When a bearing lubricated with grease is usable in lubrication with a liquid refrigerant, a circulating system of a lubricating oil and a separator of a refrigerant and a lubricating oil can be simplified, so that the circulating system and the separator can be advantageously made compact, light-weighted and highly efficient. However, since grease for lubricating a bearing is washed away from the bearing in a refrigerant atmosphere, lubrication with grease has been considered extremely difficult. It should be noted that Patent Literature 3 discloses use of grease containing MoS2 in lubrication with a liquid refrigerant. However, compositions of a main component of the grease are unclear.